As the integration density of integrated circuit devices continues to increase, it may become increasingly difficult to provide high density conductive interconnections between the active devices in the integrated circuit. In order to provide high performance interconnections, tungsten is often used as an interconnection material. The tungsten layer may be formed by Chemical Vapor Deposition (CVD). CVD tungsten can have high performance characteristics compared to aluminum or polycide.
Tungsten is generally chemical vapor deposited using a source gas containing tungsten and a reducing gas. A typical source gas used is tungsten hexafluoride (WF.sub.6). The reducing gas may be hydrogen and/or silane (SiH.sub.4). The deposition of tungsten may be obtained by the following reactions: EQU WF.sub.6 +3H.sub.2.fwdarw.W+6HF EQU 2WF.sub.6 +3SiH.sub.4.fwdarw.2W+3SiF+6H.sub.2
Unfortunately, a tungsten layer grown using the above CVD reactions may grow too rapidly. During this rapid growth, the tungsten may tend to grow in a crystalline manner, so that it grows preferentially in one direction compared to other directions. This rapid and/or preferential growth may cause the surface of the tungsten layer to be rough. The roughness of the tungsten surface may cause adhesion problems during subsequent microelectronic processing and may cause other problems that can also impact performance during subsequent processing.
One technique for reducing the surface roughness of tungsten is described in a publication by Tsai et al. entitled "Layer Tungsten and Its Applications for VLSI Interconnects", IEDM Technical Digest, 1988, pp. 462-465. As described, tungsten grain growth is interrupted using a layer of thin silicon and the silicon layer is consumed during subsequent tungsten deposition, to thereby form tungsten films with fine grains. Unfortunately, this technique interrupts the tungsten deposition process with a silicon deposition process, and may thereby complicate processing.
It is also known to provide an antireflective coating to reduce the roughness of the surface of the tungsten layer. The antireflective coating may also increase the processing complexity.